References voltages used in circuits such as memories should be stable and immune from temperature and power supply variations. A bandgap reference circuit is commonly used in memory circuits, such as dynamic random access memory (DRAM), as a reference-voltage circuit. The bandgap circuit generates a constant reference voltage that is independent of temperature and power source voltage.
The operating principle behind bandgap reference voltage generation is familiar to those in the art but is briefly described below. The bandgap reference voltage generator usually has an output voltage around 1.25 V, close to the theoretical bandgap of silicon at 0 K. The voltage difference between two unequal size diodes is used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current is used to generate a PTAT voltage in a second resistor, which is added to the voltage of a diode, in some implementations. If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the diode voltage and the PTAT voltage will be canceled out. The resulting voltage is around 1.25V. The voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts. Examples of prior art bandgap circuits that operate in this manner are provided in U.S. Pat. No. 6,788,131 to Huang and U.S. Pat. No. 5,200,273 to Mao, the entirety of which are hereby incorporated by reference herein.
In summary, the output voltage is made substantially invariant with regard to temperature by taking a weighted sum of a voltage that has a negative temperature coefficient (viz the voltage across the PN junction) and one that has a positive temperature coefficient. However, the bandgap reference voltage generator is always “on” or “enabled” in order to provide the reference voltage for the integrated circuit, thereby increasing the power consumption of the integrated circuit. These bandgap reference circuits are not appropriate for low power DRAM, which demands a low current consuming reference circuit generator.
Low power DRAM have a VDD voltage that is below 1.8V. If this VDD voltage is the power supply for the bandgap reference circuit, it cannot provide a bias independent reference voltage (Vref). That is, Vref will roll off when VDD is lower, for example, below 1.6V. The bandgap reference circuit will consume about 20 μA. In DRAM circuits there is a higher pseudo power supply voltage VH that is available, which is the power supply for the word line. This voltage can be used as the power supply for the bandgap circuit, however the bandgap circuit will consume 80 μA since the electrical current pumping efficiency is only about 25%. Pumping efficiency means the electrical current pumping efficiency. A pumping circuit will sink the power supply Idd current and generate a pumping current Ipump. The pumping efficiency is equal to Ipump/Idd.
There are two principal methods of reducing the sinking current of the bandgap reference circuit. One method is known as a the scan method. In this method, the bandgap reference circuit uses the VH supply as the power supply during the initial power-up stage. The approach uses a resistor divider with multiple switches to select the divided voltage. When the external power supply is stable (for example, when the mode register set (MRS) command is issued by the system), the switches are scanned and stop when the divided voltage is equal to the reference voltage Vref from the bandgap reference circuit. The bandgap reference circuit is then turned “off”, thereby limiting its current consumption of the bandgap reference circuit. The voltage divider supplies the reference voltage Vref for the device and consumes only a few micro-amperes of current. The penalty for this approach lies in its circuit complexity, which requires a large number of comparators to compare the divided voltage with the bandgap reference voltage supplied by the bandgap reference circuit.
The other method of reducing the sinking current of the bandgap reference circuit is to provide just enough power to the bandgap reference circuit. One such approach is shown in block diagram form in FIG. 1. A charge pump circuit is used to supply the power to the bandgap circuit. Assuming that the pumping efficiency is 50% to sustain a 2V output and the bandgap circuit pumps 10 μA (Ipump), then the circuit sinks VDD 20 μA to have the 10 μA supply capability. An oscillator is used to drive the charge pump circuit and it sinks about 5 μA. The total sink current for this approach is, therefore, about 25 μA.
In a slight modification to this approach, rather than use the charge pump output from power-up to power stabilization, the bandgap circuit uses the VH pseudo power supply during the power-up stage. As noted above, the VH supply is designed in the DRAM circuit to supply the power of the word line. When MRS is issued from the controller after power supply stabilization, the power supply of the bandgap circuit is switched to the output of the high efficiency charge pump. Since VH is a higher voltage level and thus lower pumping efficiency, it is replaced after MRS is issued to save power.
Lower power reference voltage generation circuits and methods are desired.